Therapeutic optimization in head and neck radiotherapy planning: Advocacy for FDG PET-CT in treatment condition.

Introduction: The necessity to perform FDG PET-CT both for initial tumour staging and for target volume delineation in head and neck cancers seems well established. The aim of the present study is to advocate the place and role of FDG PET-CT acquired in planning treatment position (FDG PET-CT/RT).

Methods: Between March 2018 and July 2019, 22 patients with a squamous cell head and neck carcinoma treated by EBRT were included in the analysis. All these consecutive patients had a FDG PET-CT/RT. Three GTV volumes were defined. First, "GTV 40%" corresponded to 40% of SUV. "Visual GTV" was defined as the tumor volume obtained from the PET the nuclear medicine physician interpreted. The radiation oncologist used the medical record, clinical anatomy, CT simulation and FDG PET-CT/RT data ("GTV40%" and "visual GTV") to draw the GTV.

Results: Mean GTVs and mean "GTVs40%" were significantly different (P<0.001) with an intraclass index of 0.734. Mean "GTV40%" and mean "visual GTVs" were also significantly different (P<0.001) with an intraclass index of 0.72. Conversely, the difference between mean GTVs and mean "visual GTVs" were not significant (P=0.11) with an intraclass index of 0.91. Mean DICE between "GTVs40%" and GTV was 0.7 (ranging from 0.2 to 0.9). The mean intersection between GTVs and "visual GTVs" volumes was 0.8 (ranging from 0.4 to 1). The difference between DICES was significant (P=0.015), "visual GTV"/GTV DICE was the smallest.

Conclusion: FDG PET-CT/RT definitely remains the imaging modality that individualized/customized head and neck cancer treatment needs.